1. Field of the Invention
The invention pertains to the sensing of matter in contact or in close proximity to a surface. One specific area of interest is in human interfaces, such as switches, keys, and keyboards, used for entry of data or for control of an apparatus or process. Another specific area of interest is the sensing of inanimate matter such as powders and fluids, as is done in the sensing of the level or volume of a fluent material in a container.
2. Background Information
The invention employs capacitance sensing, and in particular a form of sensing known as xe2x80x98charge-transferxe2x80x99 (or xe2x80x98QTxe2x80x99) sensing which has been taught by the inventor in his U.S. Pat. No. 5,730,165, the disclosure of which is herein incorporated by reference. Charge transfer sensing uses electronic switch closures to induce a charge onto an electrode. A disturbance in the resulting electric field is sensed by measuring the amount of charge on the electrode and to thereby determine the change in capacitance at the electrode.
Caldwell et al., in U.S. Pat. No. 5,572,205, teach a capacitive touch control system that is responsive to a user input selection and that can be configured as a touch pad disposed on an electrically non-conducting substrate, such as glass ceramic electrical cook-top. A source signal having a primary frequency that is greater than 150 kHz, and preferably in the range of between 150 kHz and 500 kHz, is applied to one portion of their touch pad. The touch pad couples the electrical signal to another portion of the touch pad in order to develop a detection signal, which is decoded in order to determine the presence of the capacitance of a user. The decoder preferably includes a peak detector composed of a low gain circuit in order to avoid distortion of the detection signal.
In the present invention two or more electrodes are arranged to create an electric field transmitted through an adjacent dielectric which can be disturbed by the proximity of an object A charge transfer measurement circuit is connected to one of the electrodes.
Because one of the major anticipated applications of the invention is in keyboards used in data entry, the sensing elements are sometimes hereinafter referred to as xe2x80x98keysxe2x80x99. It will be understood that this is done to simplify the presentation and to avoid reciting lists of known sensing or switching products that could employ the invention, and that xe2x80x98keyxe2x80x99, when so used, represents a proximity detection zone for any possible application.
Thus, one aspect of the invention is the provision of apparatus and method for detecting proximity to an electrode pair to form a key. Another aspect of the invention is the provision of apparatus and method for detecting proximity to one or more of a matrix of electrodes so as to form a keypad, keyboard, slider switch analog, or level sensor.
The creation of a key matrix follows from the arrangement of a plurality of electrode pairs, where one of each pair is connected to a voltage drive source and the second of each pair is connected to a charge detector. In the general case for a matrix there are X drive lines and Y charge detectors. Although a minimal matrix could comprise two drive lines and a single charge detector, or vice versa, an N by M matrix is expected to usually involve at least four keys; e.g., an X=2, Y=2 arrangement. It may be noted that arrangements having the same number of drive lines as they do charge detectors (hereinafter referred to as xe2x80x9csquare matricesxe2x80x9d) are generally preferred because these yield the greatest number of keys for a given amount of circuitry and wiring. It may be noted that the terms xe2x80x98matrixxe2x80x99 and xe2x80x98squarexe2x80x99 have nothing to do with the physical form of the key matrix. The keys can be arrayed linearly, circularly, or randomly on a single surface, or in any fashion desired on a plurality of surfaces. Moreover, the keys do not have to be the same physical size or shape; some can be large and circular, other small and triangular, others medium and rectangular.
In one embodiment of the invention, each key is associated with an alternating voltage source and a pair of electrodes mounted on or within a solid dielectric substrate or panel so as to be separated from each other by a gap. The voltage source is connected to a first electrode, a sampling charge detector is connected to the second electrode, and the output of the charge detector is, in turn, fed into a signal processing means. Disturbances in coupling between the two electrodes are detected through the solid substrate or panel material when a substance or object approaches or contacts the panel.
In another embodiment of the invention, each key is associated with an alternating voltage source and a pair of electrodes so as to be separated from each other by a gap. The voltage source is connected to a first electrode, a sampling charge detector is connected to the second electrode, and the output of the charge detector is, in turn, fed into a signal processing means. Disturbances in coupling between the two electrodes are detected when a substance or object approaches or contacts the electrode set directly, without an intervening solid dielectric layer.
In operation of a preferred embodiment of the invention an alternating voltage source is connected to a first (X) electrode that projects a time-varying e-field across a gap. This field is, at least in part, received by a second, receiving (Y), electrode. The receiving electrode is connected to a sampling charge detector which acts to sample the change in the charge coupled across the gap caused by the dV/dt of the pulsed voltage. It is a feature of the invention that the receiving electrode is a low-impedance node during the sampling phase of the process. This ensures that the charge coupled to the second electrode does not cause an appreciable voltage rise on the second electrode.
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and who wishes to learn how to practice the invention, it will be recognized that the foregoing recital is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore recited features and advantages of the invention, and that less than all of the recited features and advantages may be provided by some embodiments.